Most communication systems operating in the radio frequency (RF) domain have improved sensitivity and increased channel capacity based on heterodyne detection over existing direct detection. Unlike direct detection, heterodyne detection uses a local oscillator to extract the information from an input signal. At present, optical communication systems tend to use the simpler direct detection. The performance of a heterodyne detection system is the based on the accuracy of the local oscillator. For optical systems, current methods to generate an optical LO involve slow, 100 us to 1 ms, electronic, temperature, or mechanical tuning methods, which require complex feedback controls and wavelength referencing for oscillator wavelength accuracy. Alternatively, the optical systems use the flexibility of a Mach-Zehnder modulator to generate tunable sidebands on an optical carrier and then filter out the unwanted signals, such as unwanted sideband signals generated during modulation.
Methods to quickly, accurately, and reliably tune an optical local oscillator over the entire erbium doped optical fiber amplifier bandwidth are of importance to many optical systems. Local oscillators provide fine wavelength referencing in dense wavelength division multiplexed systems, signal down conversion in photonic signal processing, and frequency hopping in optical code division multiple access systems. Several techniques exist for the generation of tunable optical signals. Such techniques include the quasi-continuous tunable fiber ring laser and the generation of optical single sideband suppressed carrier signals using Mach-Zehnder modulators driven at quadrature.
Previous filtering approaches have relied on canceling signals that are in quadrature, a process that depends on careful amplitude and phase balance to suppress the unwanted signals. It is extremely difficult to maintain this signal cancellation over a wide frequency tuning range. The level of suppression of the unwanted signals is low and is frequency dependent using a Mach-Zehnder modulator that can have higher levels of suppression and can be made independent of frequency. While state-of-the-art Mach-Zehnder modulators have exhibited bandwidths up to 80 GHz, units with 60-GHz bandwidths can be purchased from commercial vendors. However, the signal phase for unwanted signal cancellation cannot be precisely maintained over this wide bandwidth. The Mach-Zehnder modulator cannot maintain an accurate phase for unwanted signal cancellation over a wide bandwidth. These and other disadvantages are solved or reduced using the invention.